1. Field of the Invention
The present invention relates to a semiconductor device, and more particularly, to a MOSFET anti-fuse.
2. Background Art
In the field of data storage, there are two main types of storage elements. The first type is volatile memory that has the information stored in a particular storage element, and the information is lost the instant the power is removed from a circuit. The second type is a nonvolatile storage element, in which the information is preserved even with the power removed. In regards to the nonvolatile storage elements, some designs allow multiple programming, while other designs allow one-time programming. Typically, the manufacturing techniques used to form nonvolatile memories are quite different from a standard logic process, which dramatically increases the complexity and chip size.
Complimentary Metal Oxide Semiconductor (CMOS) technology is the integration of both NMOS and PMOS transistors on a silicon substrate (collectively know as MOS field effect transistors, or MOSFETs). The NMOS transistor consists of a N-type doped polysilicon gate, a channel conduction region, and source/drain regions formed by diffusion of N-type dopants in the silicon substrate. The channel region separates the source from the drain in the lateral direction, whereas a layer of dielectric material that prevents electrical current flow separates the polysilicon gate from the channel. Similarly, the architecture is the same for the PMOS transistor, except a P-type dopant is used.
The dielectric material separating the polysilicon gate from the channel region, henceforth referred to as the gate oxide, usually consists of the thermally grown silicon dioxide (SiO2) material that leaks very little current through a mechanism, which is called Fowler-Nordheim tunneling under voltage stress. When stressed beyond a critical electrical field (applied voltage divided by the thickness of the oxide), the transistor is destroyed by rupturing of the oxide.
Thin oxides that allow direct tunneling current behave quite differently than thicker oxides, which exhibit Fowler-Nordheim tunneling. Rupturing oxide requires determining an appropriate pulse width duration and amplitude to limit power through the gate oxide, which produces a reliable, low resistance anti-fuse.
What is desired is an anti-fuse structure that overcomes the variability of fuse resistance. Fuse resistance is much lower at edges of the source and drain regions because it is a function of breakdown spot position along the channel region. Hence, a MOSFET device without a conventional channel region is desirable to implement an anti-fuse.